The invention relates to electronic circuits and, more particularly, to a low-pass filter that can be easily manufactured to precise bandwidth specifications and that has a changeable, programmable bandwidth for use in applications such as a threshold detector for a radio signal data slicer.
Low pass filters are circuits used in many signal processing applications, including many different types of wireless or radio telecommunications. For example, cordless telephones require wireless communications between a first and second unit such as a handset and base. The communications are often in the form of an analog data signal passed back and forth between the two units. Typically, the analog data signal utilizes a single center frequency by allocating alternating time intervals for transmission from each unit. Both units include a receiver circuit and a transmitter circuit for receiving and transmitting the analog data signals, respectively, and the low-pass filter is commonly used in a data slicer of the receiver circuit. In particular, the low-pass filter is used to determine a threshold, or "middle", voltage level of the signal by taking a long term average of the signal. In this way, the data slicer can interpret excursions of the analog data signal below the threshold level as digital "0" symbols, and those above the threshold level as digital "1" symbols. Such operation of a low-pass filter in a data slicer is described in U.S. application Ser. No. 08/864,651 (Attorney Docket No. TT1069, by Alan Hendrickson), entitled "Data Slicer for Demodulated Binary FSK Signals in a Time Division Duplex Radio Environment", which is hereby incorporated by reference.
Referring to FIG. 1, an analog data signal 10, such as may be used in the cordless telephone example described above, has certain inherent characteristics. For one, the analog data signal 10 is separated into discrete time frames, such as time division duplex ("TDD") frames 12 and 14. Each of the frames includes a transmission interval TX, during which the first unit transmits data to the second unit, and a reception interval RX, during which the first unit receives data from the second unit. As seen by a receiver circuit of one of the units (not shown), the analog data signal 10 contains "invalid", or meaningless, data during the transmission intervals TX and may contain "valid" data during the reception intervals RX. Groups of valid data in the analog data signal 10 are referred to as signal components, such as the signal component 16, and signal components are grouped in sequences. A sequence of signal components may start ("initial operation") at the beginning of a reception interval RX (such as the signal component 16 illustrated in FIG. 1), or at a midpoint of the reception interval (not shown).
These characteristics of the analog data signal 10 complicate the determination of the threshold level for the signal. For one, invalid data, such as during the transmission interval TX or during the reception interval RX when no signal component exists, adversely affects the determination of the threshold level. During the transmission interval TX, the analog data signal 10 may be near a zero-level voltage, for power-saving purposes. This zero-level voltage dramatically affects the long term average of the analog signal, and thus the threshold level. In addition, during the reception interval RX when no signal component exists, the analog data signal 10 may drift, thereby adversely affecting the threshold level. Therefore what is need is a low-pass filter that can be responsive to whether or not valid data exists in the analog data signal.
Furthermore, certain signal components of the analog data signal 10 have characteristics that are directly beneficial in determining the threshold level. For example, in normal operation, several signal components are received sequentially. Due to their location in the sequence, certain characteristics of the signal components are known or can be readily determined. For example, the first of a sequence of signal components is always strictly "balanced", i.e., it does not have long series of "1's" or "0's". In addition, other signal components in the sequence are strictly balanced, some are balanced statistically, and some are not balanced at all. Unbalanced signal components are unreliable for the determination of the threshold level, while balanced signal components are particularly advantageous for determination of the threshold level. Therefore, what is need is a low-pass filter that is responsive to the characteristics of the signal components of the analog data signal 10.
Further still, the analog data signal 10 may be distorted due to the operation of the transmitter circuit. Such distortion may be a result of phase lock loop ripple, or other well understood types of demodulator distortion. The analog signal 10, therefore, may contain distortion in the form of a slow varying component, as illustrated by reference arrow 18. Distortion in the analog data signal 10, such as the slow varying component 18, is problematic for determining the threshold level of the analog data signal 10, and therefore what is need is a low-pass filter that is responsive to slow varying components of the analog data signal.